Resonators based on piezoelectric properties of materials have been used in many important applications. For instance, quartz crystal resonators are widely used as frequency control elements in oscillator circuits found in many devices such as computers and watches. They are also used as bulk-acoustic wave filters in a variety of circuits for frequency selection purposes.
One important application of piezoelectric resonators is in detecting very small quantities of materials. Piezoelectric resonators used as sensors in such applications are sometimes called "micro-balances." A piezoelectric resonator is typically constructed as a thin planar layer of crystalline piezoelectric material sandwiched between two electrode layers. When used as a sensor, the resonator is exposed to the material being detected to allow the material to bind on a surface of the resonator.
The conventional way of detecting the amount of the material bound on the surface of a sensing resonator is to operate the resonator as an oscillator at its resonant frequency. As the material being detected binds on the resonator surface, the oscillation frequency of the resonator is reduced. The change in the oscillation frequency of the resonator, presumably caused by the binding of the material on the resonator surface, is measured and used to calculate the amount of the material bound on the resonator or the rate at which the material accumulates on the resonator surface.
The sensitivity of a piezoelectric resonator as a material sensor is typically proportional to its resonance frequency. Thus, the sensitivities of material sensors based on the popular quartz crystal resonators are limited by their relatively low oscillating frequencies, which typically range from several MHz to about 100 MHz. The recent development of thin-film resonator (TFR) technology has produced sensors with significantly improved sensitivities. A thin-film resonator is formed by depositing a thin film of piezoelectric material, such as AlN or ZnO, on a substrate. Due to the small thickness of the piezoelectric layer in a thin-film resonator, which is on the order of several microns (.mu.m), the resonant frequency of the thin-film resonator is on the order of 1 GHz or higher. The high resonant frequencies and the corresponding high sensitivities make thin-film resonators useful for material sensing applications.
The conventional method of detecting material by measuring a change in the oscillation frequency of the sensing resonator requires the incorporation of the sensing resonator in an oscillator circuit to drive the sensing resonator into oscillation. To obtain accurate measurement results, the oscillator circuit has to be stable and frequency matched to the resonant frequency of the sensing resonator. This requirement, however, is difficult to satisfy in practice. Many applications use sensors of a disposable type, i.e., sensors have to be replaced from time to time. In such a case, the same electronics will be used with many sensing resonators. Nevertheless, due to variations in the fabrication process, the sensing resonators may have significantly different resonant characteristics. For instance, the non-uniformity in the deposition thickness of a piezoelectric layer deposited across a substrate can cause the resonance frequencies of thin-film resonators from the same production batch to vary significantly. As a result, a non-adjustable oscillator circuit is incapable of effectively driving all of the sensing resonators into oscillation. It is possible to use external tuning elements to fine tune an oscillator circuit to match the resonant characteristics of individual sensing resonators. The use of tuning elements, however, can significantly increase undesirable phase noise. Moreover, fine tuning the oscillator circuit to match the sensing resonators is not feasible in practice for field applications.
Another significant disadvantage of the conventional approach is the difficulty in separating the real signal from spurious environmental effects. During material detection, a sensing resonator is often exposed to different environmental conditions that also tend to alter the oscillation frequency of the resonator. It is often difficult to isolate the frequency change caused by the material detected from the frequency changes caused by various environmental conditions.